(a) Field of the Invention
The present invention relates to an illumination optical system for an endoscope.
(b) Description of the Prior Art
Known illumination optical systems for endoscopes are generally composed as shown in FIG. 1. That is, an illumination lens 3 having a concave surface with strong curvature on the light guide side and having a strong diverging function is arranged in front of, i.e., on the object side of, the light exiting surface of a light guide 1 so that the light coming out from the light guide diverges to a wide area. However, the above-mentioned kind of illumination optical systems have a problem described below. That is, as the diverging angles of rays become larger toward the marginal portion of the illumination lens, the density of illumination rays becomes considerably low in the marginal portion of the illumination field and, consequently, the intensity of light in the marginal portion of thereof becomes considerably low. Besides, the rays that come out from the light exiting surface of the light guide at large angles in respect to the optical axis are scattered and lost by causing diffused reflection at the inner peripheral surface of the illumination lens or return to the light guide side without going out from the illumination lens by causing total reflection at the marginal portion of the front surface of the illumination lens. As a result, the brightness of illumination in the marginal portion of the illumination field becomes largely different from the brightness in the central portion thereof, and it is immpossible to obtain uniform illumination over the whole illumination field.
To solve the above-mentioned problem, it is proposed to adopt an aspherical surface for an illumination lens. For example, Japanese published unexamined utility model application No. 17071/82 discloses an illumination optical system for an endoscope provided with an illumination lens which is arranged that the surface on the light guide side is formed as a complex surface, which has a central portion formed as a spherical surface and a marginal portion formed as a conical surface, and the surface on the object side is formed as a planar surface. However, in case of said known illumination optical system the above-mentioned problem is not solved satisfactorily, and the problem of lack of uniformity of illumination still remains.
Now, for known illumination optical systems for endoscopes, detailed description is given below regarding the problem of lack of uniformity of illumination, especially, regarding the problem that the intensity of light in the marginal portion of the illumination field becomes considerably different from the intensity of light in the central portion thereof.
The conventional illumination optical systems shown in FIG. 1 comprise illumination lenses having the numerical data shown below.
Conventional lens 1
______________________________________ r.sub.1 = .infin. d.sub.1 = 0.4545 n.sub.1 = 1.883 .nu..sub.1 = 40.78 r.sub.2 = 1.1818 ______________________________________
Conventional lens 2
______________________________________ r.sub.1 = .infin. d.sub.1 = 0.4545 n.sub.1 = 1.883 .nu..sub.1 = 40.78 r.sub.2 = 1.045 ______________________________________
In the numerical data shown in the above, reference symbols r.sub.1 and r.sub.2 respectively represent radii of curvature of the first and second surfaces of the illumination lens, reference symbol d.sub.1 represents the thickness of the illumination lens, reference symbol n.sub.1 represents the refractive index of the illumination lens, and reference symbol .nu..sub.1 represents Abbe's number of the illumination lens.
Here, a ray which goes out from the light guide 1 in the direction parallel with the axis of the light guide is considered (said ray is hereinafter referred to as the principal ray) and, when the height of the principal ray is represented by reference symbol h, the angle formed by the principal ray after going out from the illumination lens in respect to the optical axis is represented by reference symbol A(h).
In case of the known illumination system shown in the above, A(h) sharply becomes large according to increase of h as shown in FIG. 2. This means that the density of principal rays becomes low in the marginal portion of the illumination field of the endoscope and, therefore, the brightness of illumination becomes low in the marginal portion. Here, the rays which go out from an optical fiber in the light guide is represented by the principal ray because, out of those rays, a ray which is closer to the state that the ray is parallel with the axis of the light guide has higher intensity of light.
Besides, when A(h) becomes considerably larger as the height of ray becomes higher (i.e., as h becomes larger), rays are largely refracted by the marginal portion of the illumination lens. This causes such phenomenon that rays partially come to the inner peripheral surface of the illumination lens and are thereby diffused and such phenomenon that incident angles of rays on the marginal portion of the front surface of the illumination lens become large and those rays partially return to the light source side by causing total reflection. As a result, the decrease of the intensity of light in the marginal portion is further promoted.